1. Field of the Invention
The present invention relates to a focus detection apparatus of a phase-difference detection method and an imaging apparatus including the focus detection apparatus.
2. Description of the Related Art
A solid-state image sensor using a photo-electric conversion element is used in various fields. For example, as an automatic focus detection method of a camera, a phase-difference detection method is generally known. In the phase-difference detection method, light rays which come from an object and have passed through different exit pupil regions of an imaging lens are imaged on a solid-state image sensor. The solid-state image sensor includes a plurality of line sensor pairs. The control means of a focus detection apparatus calculates relative positions of a pair of object images obtained by photo-electric conversion (phase-difference calculation), thus detecting a defocus amount.
When an accumulation period (photo-electric conversion period) of the solid-state image sensor is prolonged, dark current noise is generated, thus impairing focus detection precision in a low-luminance environment. Thus, a technique for correcting this dark current noise generated by a photo-electric conversion element has been proposed.
For example, in Japanese Patent Laid-Open No. H3-10473, a dark current detection unit which is not irradiated with light is arranged on a part of a line sensor, and a ratio of dark current components generated by respective pixels of the line sensor and those generated by the dark current detection unit is stored in advance. Then, the outputs from the respective pixels are multiplied with a stored coefficient to calculate dark current components of the respective pixels, thus attaining dark current correction.
Also, each photo-electric conversion element has a limitation on a voltage that can be accumulated (to be referred to as a saturated voltage hereinafter). When this saturated voltage is exceeded, overflowed charges are leaked to other adjacent photo-electric conversion elements. Thus, in Japanese Patent Laid-Open No. 2000-12820, an overflow drain switch is connected to each photo-electric conversion element, and overflowed charges are flowed toward the circuit side via this switch, thus preventing the charges from leaking. Since saturated voltages suffer manufacturing variations for respective elements, a function of adjusting a gate voltage of each overflow drain switch is provided.
However, with the technique disclosed in Japanese Patent Laid-Open No. H3-10473, an average dark current generated by the photo-electric conversion elements can be removed, but dark current shot noise as random components cannot be corrected. It is effective to reduce the dark current shot noise by decreasing an absolute value of a generated dark current amount.
FIGS. 20A to 20E are explanatory views of the technique disclosed in Japanese Patent Laid-Open No. 2000-12820. A gate OFF voltage of a transfer switch 2 is adjusted for each switch so as not to exceed a depletion voltage of a photodiode 1 and to set a sufficiently high saturated voltage. However, when the saturated voltage is high, a difference between the gate voltage of the transfer switch 2 as the overflow drain switch and a cathode voltage of the photodiode 1 (gate-source voltage Vgs) is increased. FIG. 21 shows the relationship between a dark current Idk generated by the transfer switch 2 and the voltage Vgs. As the voltage Vgs is higher, the dark current Idk to be generated increases exponentially. That is, when the gate OFF voltage of the transfer switch 2 is set to increase the saturated voltage, dark current noise is increased in a low-luminance environment, thus impairing the SN of a signal obtained by each photo-electric conversion element.